An important requirement especially in the recycling of plastics is that the end products made from the recycled material be of attractive quality and as close as possible to products made from new goods. The sorted or recycled material and the products made from it must fulfil the necessary mechanical requirements and standards and, in particular, not be impaired by noxious odors, colors, or migrating toxins, etc. Especially plastic packages that are intended for use with foodstuffs must meet strict food law rules.
If such materials are taken from the recycling pathways, they must therefore be appropriately cleaned and processed. Besides various cleaning and washing processes, which mainly clean the surfaces, often other cleaning needs to be done, especially since most polymers are not “dense” materials and migrations into the material can occur. These migrations, however, cannot be removed with washing processes acting merely on the surface or exterior.
The second step is the detachment of the contaminants from the surface of the material. The detachment is facilitated by a rinsing medium and is dependent on the ambient pressure, or facilitated by applying a vacuum. An effective detachment can only be achieved with a vacuum under 100 mbar.
In a third step, the elimination, the contaminants must then be transported away by the rinsing medium and removed from the receiving tank.
Thanks to the invention's choice of an appropriately high temperature, the providing of an appropriate vacuum and the introducing of a rinsing medium, a good efficiency of the detoxification can be achieved. Yet still one must consider other factors, especially the economy and the duration of the process.
It is basically permissible to think that one should conduct the largest possible quantities of rinsing medium through the material in order to detach and transport away the presumably large quantity of contaminants. Such methods, in which large quantities of scouring gas are conducted through the material, for drying for example, are also described in the prior art. However, in these methods, usually no vacuum is applied, and instead the methods are carried out in open containers under ambient pressure. But because of the lack of a vacuum, both the diffusion and the detachment of the contaminants is reduced. Hence, a large quantity of rinsing medium by itself is not helpful.
In this context, it should be noted that introducing a rinsing medium—and especially a large quantity of a rinsing medium—into an evacuated container always necessarily entails that the vacuum is greatly impaired or reduced by this. The more rinsing medium is introduced, the higher the pressure and the less the vacuum becomes, and the less migration and evaporation or detachment of contaminants. If a corresponding vacuum is to be maintained, it would no longer be possible to introduce sufficient rinsing medium without reducing the vacuum, so that although more contaminants would be present on the surface, still they would not be detached and thus neither could they be removed.
On the other hand, or to counterbalance this, neither can the vacuum simply be increased at will so as to achieve in this way the greatest possible migration as well as a good detachment of the contaminants. This would require a higher suction power of the vacuum pump, which in turn would entail higher overhead and operating costs. But even with the best vacuum systems, a sufficiently deep vacuum cannot be created with high quantities of rinsing medium.
Therefore, one cannot use a large quantity of rinsing medium to supposedly accomplish a high detachment and a large elimination of the contaminants without also paying heed to the vacuum. In fact, this would even be counterproductive since, as explained, it would either lower the vacuum, which in turn would mean that the detoxification efficiency is reduced on account of the worse diffusion and less detachment, or it would no longer be possible to carry out the process, or not in an economical manner.
Thus, one must balance out the countervailing parameters against each other and bring them into a harmony. In this way, one can then also determine the required dwell time of the material in the receiving tank.
The cleaning action is boosted on average by 5 to 65% as compared to a treatment without rinsing medium. It has been found that contaminants with higher evaporation points experience a greater cleaning boost than contaminants that are easy to evaporate or otherwise fundamentally easy to clean. The obtained products fulfil all required standards, e.g., they comply with ILSI, are FDA certified or certified in compliance with EFSA.
Surprisingly, it has been found in the experiments underlying the invention that, contrary to the former view, no large quantities of rinsing medium are needed and even very small quantities of rinsing medium are sufficient to drastically boost the efficiency of the detoxification. As a result, with further effective detachment of contaminants from the surface, the maintaining of the necessary vacuum becomes substantially easier and more economical. The method thus becomes more economical and the process time is shortened, or the dwell time in the reactor can become shorter.
Further advantageous configurations of the invented method are described in the dependent claims.
Thus, for an efficient diffusion or migration of the contaminants from the interior of the materials to the surface, as well as for a good detachment, it is advantageous to provide that the vacuum is held permanently below 50 mbar, in particular, between 10 and 20 mbar, preferably under 2 mbar.
According to one advantageous configuration of the method, it is possible to uses as the rinsing medium a gaseous substance, especially air, carbon dioxide, steam, an inert gas such as nitrogen. Gaseous rinsing agents can also be used in particular for an additional drying of the materials in the receiving tank. Basically all inert gases are especially useful, since they exert little or no influence on the polymer in the hot environment in the pretreatment tank.
Alternatively, it is also advantageous to use a liquid as the rinsing medium, one that evaporates immediately after entering the receiving tank, especially water. Water in particular produces an especially high efficiency of detoxification and is also more effective than air in very small amounts. Basically, liquid rinsing agents are easier to stockpile and dispense. In particular, their evaporation upon entering the receiving tank brings about a change in the state of aggregation, which helps the detachment of the contaminants from the surfaces of the polymer particles occur even better. Thus, usually at least for a short time, the liquid rinsing medium produces a wetting of the surface of the material particles. Thus, the liquid rinsing medium can loosen the contaminants found on the surface. The contaminants are detached from the surface even more efficiently during the evaporation of the rinsing medium that occurs soon afterwards.
In this context, it is advantageous for the rinsing medium to be polar or apolar. Thus, depending on the nature of the polarity of the contaminants, the most efficient solubility or mixture of the contaminants in the rinsing medium can be achieved.
According to one advantageous process, the rinsing medium is heated and/or dried before entering the receiving tank, e.g., by an upstream heating appliance or a gas drying appliance. The heating is done in energy-saving manner, advantageously from the waste heat of the overall process.
In the case of liquid media, the energy of evaporation is advantageously extracted from the system. For this purpose, the bottom disk, which is usually configured as a cooled disk, can be provided with small nozzles and the energy of evaporation can be used for cooling of the disk.
The type of rinsing agents introduced depends on the nature and quantity of anticipated contaminants, the properties of the material being cleaned, and the stated purpose.
For example, if it is the stated purpose to clean migration products from milk bottles made of high density polyethylene (HD-PE), then in addition to the correct process temperature and the establishing of the necessary vacuum it is advantageous to introduce water or air for the intensified cleaning. Water is sprayed in in small amounts and turns into steam or gas under the process conditions. The use of water is relatively noncritical in the case of HD-PE or other polyolefins, since these polymers react relatively insensitively to water under the given process conditions, especially the elevated temperature.
But if the stated purpose is the cleaning of migration products from polyethylene terephthalate (PET) flakes and if the material properties of PET are to be preserved or even improved, then water is a conceivably unfavorable medium, since water or steam at elevated temperatures can lead to a breakdown of the molecular chain of PET. Air or oxygen can also lead to an oxidative discoloration, which is undesirable. For this reason, one will in this case prefer an inert gas or a liquid having no influence on the polymer. Moreover, ethylene glycol or diethylene glycol is produced during the polycondensation which sometimes occurs. These products are also more easily taken away. Yet a precondition is that the rinsing medium be free of moisture, e.g., dry nitrogen.
Yet there are also stated purposes in which a targeted influencing of the viscosity of PET and at the same time a carrying away of the migration products is desired, e.g., during the recovery of recycled goods in a polymerization process. In this case, a breakdown of the molecular chains is in fact desired and water, for example, proves to be a favorable rinsing medium, since it provides for an accelerated removal of the migration substances, as well as a breakdown of the molecular chains.
The main purpose of the rinsing medium, however, is the detachment of the migration products from the surface of the polymer particles, as well as the subsequent carrying away of the migration products.
According to another advantageous process, the rinsing medium enters the receiving tank or strikes the material being cleaned with a velocity of at least 1 m/min. It has been found that the second step in particular, namely, the detachment of the contaminants from the surface of the materials, is also dependent on the velocity of the flowing medium or the velocity with which the rinsing medium strikes the surface. In this way, the detachment of the contaminants is more or less physically constrained and the contaminants are entrained by the rinsing medium flowing past them. This effect occurs at a limit velocity of the rinsing medium of at least one meter per minute, or in many cases already at 0.8 or 0.9 m/min. Thus, if the rinsing medium strikes a polymer particle being cleaned with such an oncoming velocity or a higher velocity, large amounts of contaminants, already having migrated to the surface of the flakes on account of the temperature, will be carried away and taken up into the flow of rinsing medium.
According to another advantageous process, the quantity of rinsing medium entering the receiving tank in the case of a gaseous rinsing medium is in the range of 0.1 to 4.7 Nm3, (normal cubic meters Vn), preferably 1 to 3 Nm3, per hour per kilogram of material throughput per hour, and in the case of a liquid rinsing medium prior to the evaporation it is in the range of 0.0001 to 0.08 liters, preferably between 0.003 and 0.05 liters, per hour per kilogram of material throughput per hour. As a rule, the partial vacuum ranges from 2 to 20 or 50 mbar. The process can be conducted especially efficiently, quickly and economically in these ranges.
According to another advantageous process, the quantity of rinsing medium entering the receiving tank in the case of a gaseous rinsing medium is <0.050 Nm3, preferably in the range of 0.001 to 0.047 Nm3 (normal cubic meters Vn), preferably 0.001 to 0.030 Nm3 per hour per kilogram of material throughput per hour, and in the case of a liquid rinsing medium prior to the evaporation it is in the range of 0.0001 to 0.08 liters, preferably between 0.003 and 0.05 liters, per hour per kilogram of material throughput per hour. As a rule, the partial vacuum ranges from 2 to 20 or 50 mbar. The process can be conducted especially efficiently, quickly and economically in these ranges.
The method of the invention preferably runs continuously, the material being detoxified is introduced continuously into the receiving tank and after a certain dwell time it is taken out once more, e.g., to a delivery worm, an extruder, or another treatment tank, and the rinsing medium also flows in and is taken away continuously.
It is especially efficient for the rinsing medium to be introduced through the bottom surface of the receiving tank, especially a cylindrical one.
Due to the fact that the flakes in the receiving tank are swirled around in a rotating mixing vortex, the materials often sweep past the inlet opening for the rinsing medium. It is advantageous for the inlet opening to be relatively small. Thus, the materials at this site will be locally struck by the rinsing medium and the contaminants will be detached and taken up into the gas flow. After this, the particle moves onward and the next particle gets into the inflow region of the rinsing medium stream. In this way, a large amount of particles in a given time moves past that site or that stippled inlet region where the velocity or local flow of rinsing medium is more than 1 m/min. Whether the velocity is reduced in other regions of the tank, further removed from the inlet nozzle, is then a secondary concern, since the detachment has already taken place.
A preferred variant, especially for making sure to reach the necessary minimum velocity of the rinsing medium, is characterized in that the rinsing medium is introduced by at least one nozzle, if necessary a single nozzle, such as a needle nozzle, preferably with a diameter of 1 to 3 mm. The open area of the nozzle is advantageously less than or equal to 70 mm2.
Oftentimes baffle plates are placed in front of the inlet openings to prevent clogging. This is neither necessary nor advantageous in the present case, as it would prevent a direct and immediate striking of the particles by the rinsing medium. Thus, the inlet openings for the rinsing medium in the receiving tank should be as uncovered as possible and free of covers that deflect the rinsing stream or lower its velocity.
Especially when processing polymer materials it is advantageous for the flakes to remain single and loose, since only this will ensure a large overall surface of material. For this reason, it is advantageous when processing a polymer material for the treatment to occur at a temperature above the glass transition temperature and below the melting range, preferably at a temperature at which the material exists in a softened state, preferably in the region of the VICAT softening point (per DIN 306, A, 10N, 50 K/h). The flakes softened and sticky in this way are kept single and free flowing by the constant movement and blending, and a baking into a compact cake with small surface is prevented.
According to one advantageous process control, it is possible to add at least one other receiving tank or pretreatment tank upstream or downstream from the receiving tank, and to have the material run through all receiving tanks in succession. The method of the invention is preferably carried out in each of the receiving tanks, and the process control can also be varied. By appropriate choice of the parameters, adapted to the material and the anticipated contaminants, and in order to achieve an even better efficiency of the detoxification, different kinds and/or different quantities of rinsing agents can be used in the tanks. Thus, for example, for a complete removal of several contaminants at the same time, it is advantageous to use different polar rinsing agents in the receiving tanks, e.g., a polar rinsing medium in the pretreatment tank and an apolar rinsing medium in the main treatment tank, or vice versa. An advantageous device for this is described, for example, in WO 03/103915. This device, as presented in the figures and the specification of WO 03/103915, is included in the present application by reference.
In this context, it is especially advantageous to provide a pretreatment tank and a main treatment tank connected to it, while the method according to one of the preceding claims is carried out in each of the two tanks. An especially effective detoxification is achieved when the temperature in the main treatment tank is greater than that in the pretreatment tank and/or the pressure in the main treatment tank is less than that in the pretreatment tank.
Moreover, according to the invention, there is specified, or the problem is solved by a special device, in which the gas feeding means is configured such that the rinsing medium can be introduced into the receiving tank with a velocity of at least 1 m/min and the receiving tank and the vacuum pump are configured or structurally designed so that a vacuum can be maintained in the receiving tank constantly below 100 mbar. The method of the invention can be carried out quickly and easily with such a device.
The device of the invention is essentially based on a commercial device, such as is available, for example, under the name Vacurema® Basic, Advanced or Prime from the firm Erema. For example, one can use a device as specified in EP 2 117 796. The device presented in EP 2 117 796 and specifically described in the specification is taken up by reference into the contents of the present application.
In the method of the invention, the rinsing medium that is capable of detaching the contaminants, taking them up or carrying them along, is introduced from below, i.e., in a region below the level of the material located in the cutter-compactor in operation or below the level of material of the mixing vortex forming inside the cutter-compactor. The rinsing medium enriched or possibly saturated with contaminants, and then gaseous, is brought out from the cutter-compactor in a region above the level of the material located in the cutter-compactor during operation or above the material level of the mixing vortex. A forced flow is formed in this way and the rinsing medium flows through at least one subregion of the material.
The feeding means for the rinsing medium can be configured as a passive feeding means, for example, as mere through openings, through which the gas is sucked in purely passive manner into the inside of the cutter-compactor, such as by virtue of the partial vacuum. But the feeding means can also be configured as active feeding means, such as nozzles or the like, through which the gas can be actively blown, sprayed or pumped into the inside of the receiving tank, for example, with pumps, blowers, etc., using excess pressure.
The feeding means, on the one hand, can be configured in particular in the form of one or more nozzles in the bottom surface of the cutter-compactor, beneath the lowermost bottom mixing element, and preferably inside the innermost radius third of the bottom surface. If the feeding means are configured in the bottom surface, the blowing in of the rinsing medium from below also brings about a certain loosening of the sump, which ensures an even better treatment and detoxification of the material.
The feeding means can be configured as individual single openings or in the form of an annular slot opening running almost continuously around the bottom surface in order to lead through the drive shaft of the mixing element.
Alternatively or additionally to the openings in the bottom surface, the feeding means can also be arranged in the side wall of the cutter-compactor, making sure that the feeding means are always below the material level. Advantageous are positions of the feeding means in the area of the lowermost third of the overall height of the cutter-compactor, especially below the lower or lowermost mixing elements near the bottom.
In devices with several mixing elements arranged one above the other, it is advantageous for the process control to arrange the feeding means between the uppermost and the lowermost mixing elements and have them empty into the space formed between every two disks or mixing elements. In this way, the material is well exposed to the flow of rinsing medium and the flow advantageously works together with the mixing by the mixing elements.
Advantageously, the feeding means do not empty in the region of the edges of the support disks or the mixing elements, but rather in particular in the region between every two support disks or mixing elements in the receiving tank, or they are arranged there, while the feeding means are arranged in particular in the middle between every two support disks or mixing elements.
In this context, it is especially advantageous to have breakthroughs, at least in the upper support disk, since in this way the contaminants can be effectively removed from the region between the mixing elements.
If the feeding means are configured in that region of the side wall of the tank in which the rotating material particles exert the greatest pressure on the side wall, the feeding means must counteract this pressure and bring the rinsing medium by pressure into the interior of the tank as active feeding means.
The gas feeding means can also be configured as individual single openings in the side wall. But they can also be configured in the form of an annular gap extending along the circumference.
Alternatively or additionally to the possible arrangements described thus far, the feeding means can also be arranged on at least one of the mixing elements or on the support disk. An arrangement on the lower mixing element, closest to the bottom surface, or on the lower support disk is advantageous; when the feeding means is arranged on the mixing elements or support disks, it is advantageous to configure the feeding means on the side facing the bottom surface.
Furthermore, it is advantageous to arrange the feeding means close to the axis of the support disk or the mixing elements and advantageously close to the edges of the mixing elements following on behind the support disk as it revolves or close to the breakthrough. In this way, an effective removal of the contaminants can be assured.
A flowing of rinsing agents through the material in a cross current, i.e., from side wall to side wall, is likewise possible, but somewhat more expensive. However, it is advantageous when a possible recondensation needs to be prevented. In theory, a feeding of the rinsing agents from above is also possible.
To regulate the feeding and removal, the feeding means are advantageously at least partly movable and controllable. An actuating element or valve advantageously controls the quantity of rinsing agents. The actuating element can be controlled via a metering of the vacuum. This is advantageous, because the vacuum needs to be maintained in order to have an adequate diffusion gradient.
It has been shown in experiments that the cleaning effect can be boosted and the process time shortened when rinsing agents are introduced into a vacuum-tight receiving tank according to the invention. It is important that the agents also in fact flow through the material and at the same time the vacuum is further maintained.
The invention will now be described by means of especially advantageous sample embodiments, not to be taken as limitations.